1. Field of the Invention
The present invention relates to a measuring apparatus and an area quality measuring method, which measure service quality in an area by using a signal-to-interference ratio (SIR) and the like.
2. Description of the Related Art
Heretofore, a communication system (for example, a code division multiple access (CDMA) communication system), in which one frequency band is used in communication channels concurrently set between mobile stations and a base station, has been widely known in general.
Specifically, in the CDMA communication system, the plurality of communication channels concurrently set between the plurality of mobile stations and the base station are identified by using spreading codes. In such a CDMA communication system, since one frequency band is used in the plurality of communication channels set concurrently, a certain communication channel is subjected to interference from the other communication channels.
Hence, in the CDMA communication system, a ratio of reception power of a signal transmitted by using the certain communication channel, to reception powers of signals (interference signals) transmitted by using the other communication channels (a signal-to-interference ratio; hereinafter, “SIR”) needs to be maintained at or above a predetermined threshold value at which the certain communication channel can be identified with the spreading code.
Here, the number of concurrently-settable communication channels, in which the SIRs are maintained at or above the predetermined threshold value, is considered as one of indices that indicate service quality in an area. Specifically, when the reception powers of the interference signals (I) are small, the SIR is maintained at, or above the predetermined threshold value even if transmission power of the signal (S) is made small. Hence, there is room for increasing the number of concurrently-settable communication channels when the maximum transmission powers of the mobile stations and the base station are constant.
On the other hand, when the reception powers of the interference signals (I) are large, the reception power of the signal (S) needs to be large. Otherwise, the SIR cannot be maintained at, or above the predetermined threshold value. For this reason, there is no room for increasing the number of concurrently-settable communication channels when the maximum transmission powers of the mobile stations and the base station are constant.
As a method for measuring such service quality in an area by using the SIR as described above, a method using a constant power channel for use in a case of transmitting a signal with constant transmission power has been proposed (for example, Japanese Patent Application Laid-Open Publication No. 2001-36462 (Claim 1, FIG. 3, Paragraph 0031, and the like)). Note that the constant power channel is a channel for which a transmission power control is not performed.
According to this method, since the constant power channel is a channel in which the transmission power is not controlled, the reception power of the signal transmitted by using the constant power channel is only influenced by geographical factors of the concerned area.
Thus, accuracy in measuring the service quality in an area can be enhanced in comparison with a case where the service quality in the area is measured by using the reception power of the signal transmitted through the channel in which the transmission power is controlled.
Meanwhile, in the CDMA communication system, a total value of the transmission powers of the signals transmitted by the base station to the mobile stations is varied in response to the number of mobile stations concurrently communicating with the base station. In other words, when the signal transmitted by using a certain communication channel is set as a measurement target, a total of the reception powers of the interference signals transmitted through other communication channels is varied.
As a result, in the above-described method, when the SIR is measured while setting the signal transmitted by using the certain communication channel to be the measurement target, the SIR is undesirably varied in response to a timing of measuring the SIR (that is, in response to the number of mobile stations concurrently communicating with the base station).
As described above, in the conventional method, the SIR is undesirably varied in response to the number of mobile stations concurrently communicating with the base station. For this reason, accuracy in measuring the service quality in an area cannot be enhanced sufficiently.
In particular, while a communication system is designed on the supposition of a case where the number of mobile stations concurrently communicating with the base station is the maximum (hereinafter, “most frequent case”), a measured SIR is often not an SIR of the most frequent case. Accordingly, it has been difficult to design the communication system efficiently.